1. Field of the Invention
The invention relates to counter rotating aircraft gas turbine engines with counter rotating fans driven by counter rotating low pressure turbine rotors and, particularly, for such engines incorporating vanes to effect unequal power splits between the counter rotating low pressure turbine rotors.
2. Description of Related Art
A gas turbine engine of the turbofan type generally includes a forward fan and booster compressor, a middle core engine, and an aft low pressure power turbine. The core engine includes a high pressure compressor, a combustor and a high pressure turbine in a serial flow relationship. The high pressure compressor and high pressure turbine of the core engine are interconnected by a high pressure shaft. The high pressure compressor, turbine, and shaft essentially form the high pressure rotor. The high pressure compressor is rotatably driven to compress air entering the core engine to a relatively high pressure. This high pressure air is then mixed with fuel in the combustor and ignited to form a high energy gas stream. The gas stream flows aft and passes through the high pressure turbine, rotatably driving it and the high pressure shaft which, in turn, rotatably drives the compressor.
The gas stream leaving the high pressure turbine is expanded through a second or low pressure turbine. The low pressure turbine rotatably drives the fan and booster compressor via a low pressure shaft, all of which form the low pressure rotor. The low pressure shaft extends through the high pressure rotor. Some low pressure turbines have been designed with counter rotating turbines that power counter rotating fans and booster or low pressure compressors. U.S. Pat. Nos. 4,860,537, 5,307,622, and 4,790,133 disclose counter rotating turbines that power counter rotating fans and booster or low pressure compressors. Most of the thrust produced is generated by the fan.
Advanced gas turbine engines having counter rotating forward and aft fans and counter rotating boosters are being designed. It is desirable to design a counter rotating engine with a peak performance. It has been found that a peak performance can be attained when the forward fan operates at a higher fan pressure ratio and higher rotational speed than the aft fan. This can result in a substantial mis-match in horsepower and rotational speed between the counter rotating rotors. The counter rotating low pressure turbine is required to supply the necessary power to each of the forward and aft fans at the rotational speed of each fan. A conventional counter rotating turbine will operate at peak efficiency when the power split between both shafts is equal and when the rotational speeds are equal and opposite. In such a case, speed and horsepower ratios of the two rotors and turbines are substantially 1. It is highly desirable to have a gas turbine engine with counter rotating low pressure turbines that have different speed and horsepower ratios such as speed and horsepower ratios of 1.2 or more to attain peak fan efficiency.
An aircraft gas turbine engine includes a high pressure rotor having a high pressure turbine drivingly connected to a high pressure compressor by a high pressure shaft and rotatable about an engine centerline. A low pressure turbine having a low pressure turbine flowpath is located aft of the high pressure rotor. The low pressure turbine includes counter rotatable low pressure inner and outer shaft rotors having low pressure inner and outer shafts, respectively, which are at least in part rotatably disposed co-axial with and radially inwardly of the high pressure rotor. The low pressure inner shaft rotor includes first low pressure turbine blade rows disposed across the low pressure turbine flowpath and drivingly connected to a first fan blade row by the low pressure inner shaft. The low pressure outer shaft rotor includes second low pressure turbine blade rows disposed across the low pressure turbine flowpath and drivingly connected to a second fan blade row by the low pressure outer shaft. The first and second fan blade rows are disposed within a bypass duct radially outwardly bounded by a fan casing. The first low pressure turbine blade rows includes at least one first interdigitated turbine blade row disposed between at least one second adjacent pair of the second low pressure turbine blade rows. The second low pressure turbine blade rows includes at least one second interdigitated turbine blade row disposed between at least a first adjacent pair of the first low pressure turbine blade rows. The low pressure turbine includes a plurality of rows of non-rotatable low pressure vanes. Each one of the rows of the non-rotatable low pressure vanes is disposed across the low pressure turbine flowpath between each non-interdigitated adjacent pair of the first and second low pressure turbine blade rows not having an interdigitated turbine blade row therebetween.
In the exemplary embodiment of the invention, at least one booster is drivingly connected to one of the low pressure inner and outer shafts and axially located between the first fan blade row and the high pressure rotor. A low pressure turbine nozzle is disposed axially forward, upstream of, and adjacent to the first low pressure turbine blade rows.
Various configurations of the low pressure turbine may be used. A forwardmost row of the second low pressure turbine blade rows may be interdigitated with an aftmost pair of the first low pressure turbine blade rows. The low pressure turbine may have an odd number of one of the first low pressure turbine blade rows or the second low pressure turbine blade rows and an even number of another of the first low pressure turbine blade rows or the second low pressure turbine blade rows. The low pressure turbine may have an odd number of the first low pressure turbine blade rows and an even number of the second low pressure turbine blade rows. The low pressure turbine may have three of the first low pressure turbine blade rows and four of the second low pressure turbine blade rows. The two forwardmost rows of the second low pressure turbine blade rows may be interdigitated with three aftmost rows of the first low pressure turbine blade rows.